This invention relates to ultrasound imaging systems, and in particular to methods and systems for simultaneously displaying image information derived from multiple imaging modes. This invention can be used to enhance the ultrasound image quality for moving or contrast agent-containing tissue structures, especially when imaging difficult-to-image patients where clutter or other stationary noise superimposed on the image makes high quality imaging of moving tissue structures difficult.
Conventional ultrasound image enhancement methods using only B-mode information may not work well in some cases where a good classification between the blood pool and moving tissue is difficult to achieve by using only the B-mode and/or processed B-mode information. Various image processing techniques have been developed to improve B-mode ultrasound images.
Lipshutz, U.S. Pat. No. 5,224,483, discloses a system for enhancing an ultrasound image by reducing clutter in a blood pool area of the image. The blood pool areas are identified using low-pass filtering and non-linear intensity mapping, and a mask image is then generated to have a first value in areas of tissue and substantially a second value in the areas of blood pool. The original image is then modulated with the mask image to substantially remove clutter in the blood pool.
Ustuner et al., U.S. Pat. No. 5,479,926, disclose another B-mode image enhancement method for combining a first image signal which has greater detail information and a second image signal which has greater contrast information, into a single image using a 2D look-up table. In one of the preferred embodiments, the second processed image can be obtained from a motion estimator which calculates the correlation coefficients between consecutive B-mode frames.
Arenson et al., U.S. Pat. No. 5,285,788, disclose a Doppler tissue imaging method (DTI) that uses color Doppler imaging means to image moving tissue so that stationary clutter can be removed or greatly suppressed since the Doppler signal is only sensitive to moving targets. The disclosed DTI imaging can output tissue velocity, energy, or acceleration as a two-dimensional image which is spatially coordinated and superimposed on a B-mode image to display simultaneously the selected Doppler information and a tomographic image of the moving tissue. For Doppler tissue velocity imaging (DTV), the moving tissue velocity is the primary parameter to be displayed. Conventionally, a color map is used to encode the direction as well as the magnitude of the velocities, and a gray scale B-mode image signal may also be partially added to provide tomographic information of the moving tissue, i.e., EQU I.sub.o.sup.RGB =C.sup.RGB (I.sub.V)+.beta..multidot.I.sub.B,
where I.sub.o.sup.RGB is color coded output image, C.sup.RGB represents the velocity color mapping function, I.sub.V represents Doppler tissue velocity, I.sub.B represents the gray scale B-mode image signal, and .beta. is a scaling factor with its value between 0 and 1.
Doppler tissue imaging provides a means to improve the moving tissue imaging with excellent stationary clutter noise suppression. In its video mix mode the Doppler signal from the moving tissue is mixed with the B-mode image, augmenting the B-mode image of moving tissue. However, the simple additive blending of the DTI image and the B-mode image does not fully utilize all the available information given by these two image signals.
In the past, various contrast agents have been used to enhance contrast of blood and perfused tissues. Typically, a contrast agent is introduced into a part of the body which is to be ultrasonically imaged. For example, in the case of a blood-filled chamber of the heart, blood which carries contrast agent can be distinctly imaged by detecting the contrast agent.
Nonlinear scattering from contrast agents is described, for example, by V. Uhlenhdorf, et al., in "Nonlinear Acoustic Response of Coated Microbubbles in Diagnostic Ultrasound" (1995 Ultrasonic Symposium, pp. 1559-1562). Such contrast agents possess a fundamental resonant frequency. When the contrast agents are insonified with a high intensity ultrasonic energy at this fundamental frequency, they reflect and radiate ultrasonic energy at both the fundamental frequency and a harmonic of the fundamental frequency. For example, if insonified at a frequency of 2.5 MHz, the contrast agent may radiate energy at both 2.5 MHz (the fundamental frequency) and at 5.0 MHz (the second harmonic frequency).
Typically, non-linear contrast agents are used with an imaging system having a transmit beamformer that transmits ultrasonic energy and a receive beamformer that receives the reflected ultrasonic energy. The transmit beamformer insonifies the area to be imaged with ultrasonic energy at a fundamental frequency. When insonified with ultrasonic energy at the fundamental frequency, the contrast agent radiates energy at both the fundamental and harmonic frequencies as described above. The receive beamformer receives both the fundamental and harmonic energy, filters out the fundamental energy, and forms a harmonic image from the received harmonic energy. Ideally, the harmonic image relates only to the scattering from the contrast agent.
The harmonic image, however, may contain harmonic frequency components related to scattering from tissues in addition to the desired harmonic energy. For example, the transmit beamformer may transmit energy at the harmonic frequency as well as at the fundamental frequency. This energy scatters linearly and is included in the harmonic image. In addition, the receive beamformer may not completely filter out energy at the fundamental frequency, so this fundamental frequency leaks into the harmonic image. Finally, non-linear scattering from tissues or non-linear propagation through tissues may result in harmonic energy being scattered from normal tissues and included in the harmonic image, even in the absence of a contrast agent.
Brock-Fisher et al., U.S. Pat. No. 5,577,505, combine a colorized non-linear image with a gray-scale image. The non-linear image is obtained via a subtraction approach, requiring insonifing the tissue at two different times and power levels. Further, the combination includes only the simple steps of colorizing the non-linear signal and summing with the gray-scale image.
Monaghan, U.S. Pat. No. 5,255,683, combines a B-mode image taken before the introduction of a contrast agent with a subtraction image formed from images taken after a contrast agent has been introduced. Monaghan, however, requires images to be acquired before and after the introduction of a contrast agent. The scan is thus not in real-time, and the scan plane must be identical for each firing before and after the introduction of the non-linear contrast agent.